A non-player character, or NPC for short, is a character that is not controlled by a human player. These characters are part of the game's system, and thus, can only be manipulated by the game server. They are often seen in towns and rarely in dungeons, and can be interacted by players to do particular tasks, particularly talking, trading and offering services, among others.
There are two major types of NPCs: friendly NPCs which are either neutral or friendly to players, or hostiles — also known as mobs — which attack players if within range.
Players can interact with NPCs by either clicking on them or pressing Q while in range of the NPC; NPCs have a light blue glow around them if a player is close enough. Depending on the available options, the NPC can either talk to the player, continue with a quest dialogue, or open a window depending on the available service of the NPC.